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The Friedel-Crafts Reactions Friedel-Crafts Alkylation (RCl + AlCl3) R3 C H H H Cl AlCl3 H R H R CR3 - H+ R benzene arenium ion alkyl halide Lewis acid complex alkylbenzene Friedel-Crafts Acylation (RCOCl + AlCl3) O O CH3 C + AlCl3 Cl CH3 C Cl AlCl3 CH3 acetyl chloride C O + acyllium ion O H C H CH3 H H H O - H+ O CH3 CH3 benzene acetophenone AlCl4 Problems with Friedel-Crafts Alkylation Carbocation rearrangements H H H H H H R R primary alkyl halide Lewis acid complex secondary carbenium ion H H H H Cl AlCl3 H H H Cl AlCl3 H R R R R primary alkyl halide Lewis acid complex tertiary carbenium ion Friedel-Crafts Alkylation with a primary halide H AlCl3 Cl Cl3Al H Cl H H H H H 2° carbocation 1-chlorohexane H 3C H H H H H H CH3 H - H+ H benzene 2° cation 2-phenylhexane Carbocation stability R R R R R R benzyl cations R > R ≅ > cyclopropylmethyl carbocations R R H > H H R R R 3° cations 2° cations 1° cations R allyl cations > An alternative method of cation formation H AlCl3 H alkene H Cl3Al H AlCl3 H H+ two separate steps - AlCl3 H phenylcyclohexane 2° cation A Small Problem with Friedel Crafts Acylation A practical difficulty, acyl benzenes are Lewis bases H O H R AlCl3 O AlCl3 R Therefore use at least one equivalent of Lewis acid in Freidel-Crafts acylation reactions. Reduction of Aryl Ketones Clemmensen Reduction H H O Zn(Hg), HCl R R or Catalytic Hydrogenation H O R H H2, Pd or Pt or other metal R Wolff-Kishner H H O H2NNH2 R R NaOH Reduction of Nitroaromatics O N H O Sn HCl nitrobenzene N H H H NaHCO3 N Cl anilinium chloride or aniline hydrochloride aniline H Mechanism of Wolff-Kishner (will not be on an exam) H O H H O H H2NNH2 R H HO N NH2 R hydrazine I H O H H OH H H H HO N N R II a hydrazone I OH H N N H H N H II N O N H H H O H III OH H N R R III N R R H H NH2 R H N H N Phenyl Diazonium Ions Diazotization of anilines HONO NaNO2 + HCl H N N2 Cl HONO H aniline benzene diazonium chloride Mechanism of diazotization H H H N H H O N O O N N O H H O H H N N H N Cl Cl benzene diazonium chloride aniline Diazo Coupling N N H N Cl N H N H -HCl N N H Sandmeyer Reaction Sandmeyer Reaction N N C Cu2CN2 N KCN X benzonitrile benzene diazonium halide Reactions closely related to the Sandmeyer Reaction Br N Cu2Br2 N Cl Cu2Cl2 X KI bromobenzene chlorobenzene I iodobenzene Replacement of diazo group by fluorine H N N NaNO2 H Replacement of diazo group by hydroxyl Cl Br Br N N Br fluorobenzene benzene diazonium fluoroborate Replacement of diazo group by hydrogen Br F ∆ BF4 HBF4 aniline N H H3PO2 N N H 2O Cl Br Br H2SO4 ∆ OH